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Problem solution · SQL

Leetcodify Similar Friends

Leetcodify Similar Friends: a SQL solution using relational aggregation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Relational aggregation
Source
walkccc LeetCode Solutions
Length
16 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Relational aggregation

For Leetcodify Similar Friends, the query transforms and combines relational rows, then filters or aggregates them into the requested result.

  1. Identify the source rows and join keys.
  2. Apply filters before aggregation when possible.
  3. Group, rank, or project the final columns required by the result.

Code notes

  • 16 lines of SQL from the credited upstream file 1919.sql.
  • The implementation keeps its working state in language-native values and containers.
  • No explicit loop blocks detected.

Complexity

Review join cardinality, grouping keys, and available indexes when estimating query cost.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeLeetcodify Similar Friends · SQLSQL
Use this to learn the idea, then write your own version.
SELECT DISTINCT  Listen1.user_id AS user1_id,  Listen2.user_id AS user2_idFROM Listens AS Listen1INNER JOIN Listens AS Listen2  USING (song_id, day)WHERE  Listen1.user_id < Listen2.user_id  AND EXISTS(    SELECT * FROM Friendship    WHERE      Listen1.user_id = Friendship.user1_id      AND Listen2.user_id = Friendship.user2_id)GROUP BY Listen1.user_id, Listen2.user_id, Listen1.dayHAVING COUNT(DISTINCT Listen1.song_id) >= 3

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